Project Summary Inflammatory bowel disease (IBD) is often neuropsychiatric disorders, such as anxiety, depression and panic disorders. However, the etiology of IBD-associated neuropsychiatric disorders remains understudied. The focus of this R21 research proposal is to define the molecular and cellular mechanisms through which changes in the intestinal microbiota influence neuronal circuits and brain health that underlie neuropsychiatric disorders associated with IBD. The microbiota is known to regulate mammalian physiology, metabolism and immune homeostasis, and dysbiosis has been linked to the pathogenesis of diverse diseases including IBD. Recently, there is increasing evidence that changes in the intestinal microbiota can have a direct impact on the brain. Studies have reported correlations between gastrointestinal abnormalities and neuropsychiatric disorders, such as autism, cerebral palsy and depression. Animal studies indicate that the absence or modification of the gut microbiota affects neurogenesis, cortical myelination, as well as a variety of behaviors, such as stress, anxiety, cognitive behaviors and panic disorders. Metabolomic studies have shown that metabolites derived from the microbiota and host metabolites altered by the microbiota may influence metabolic, immunologic, and behavioral phenotypes in mice and humans. Collectively, these studies suggest that altered microbiota in IBD patients may contribute to the neuropsychiatric comorbidities associated with IBD. In preliminary analyses, we employed treatment of wild-type mice with a cocktail of broad-spectrum antibiotics to significantly alter the microbiota and examined host behavior in the fear extinction test ? a pre- clinical model of fear-related anxiety disorders. Direct manipulation of the microbiota had a profound effect on fear extinction in these mice. Moreover, GF mice exhibited similar defects in fear extinction that was associated with significantly decreased levels of serum serotonin and dopamine, both of which are reported to play important roles in fear extinction. These findings indicate that the microbiota has an effect on host behavior in the fear extinction model. To dissect the mechanisms by which changes in the intestinal microbiota influence brain function and behavior, in Aim 1, we will employ in vivo two-photon microscopy to directly test whether manipulation of the intestinal microbiota influences neuronal synapse remodeling in mice in the context of fear extinction. In Aim 2, we will employ comparative metabolomics to test how changes in the microbiota influence the metabolite profile in mouse serum and CSF in the context of fear extinction. Using these combined approaches of manipulating the microbiota, coupled with cutting-edge methods of live neuronal imaging and metabolomic profiling, we will generate fundamental new insights into how alterations in the intestinal microbiota are associated with the neuropsychiatric comorbidities associated with IBD.